This invention relates to a device for measuring the concentration of a nitrogen oxide in a combustion gas such as an exhaust gas of an internal combustion engine. A probe part of the device has an ion pumping cell using an oxygen ion conductive solid electrolyte, a catalyst for decomposing the nitrogen oxide and an oxygen partial pressure detector such as a concentration cell using an oxygen ion conductive solid electrolyte.
A chemiluminescence analyzer has been used for measuring the concentrations of nitrogen oxides in exhaust gases of automotive internal combustion engines. The analyzer includes a reaction chamber for reacting nitrogen monoxide with ozone to form nitrogen dioxide by the process represented by the reaction formulas (i), (ii) and (iii). EQU NO+O.sub.3 .fwdarw.NO.sub.2 (or NO.sub.2 *)+O.sub.2 (i) EQU NO.sub.2 *.fwdarw.NO.sub.2 +h.nu. (ii) EQU NO.sub.2 *+M.fwdarw.NO.sub.2 +M* (iii)
where NO.sub.2 * represents NO.sub.2 in excited state, h.nu. is luminous energy, and M represents the molecule of a coexisting gas.
That is, a portion of nitrogen monoxide turns into nitrogen dioxide in an excited state which is unstable in respect of energy, and the resuming of the normal state of nitrogen dioxide is accompanied by emission of light of wavelengths (0.6-3.0 .mu.m) in the near-infrared region. This phenomenon is utilized since the intensity of the emitted light is approximately proportional to the concentraion of NO (number of NO molecules) in the gas. In the analyzer, the emitted light is amplified and transformed into an electrical signal by a photomultiplier tube to thereby measure the NO concentration in the sample gas.
Since only the concentration of NO can be determined by this method, usually it is necessary to pass the sample gas first through a converter for reduction (or thermal decomposition) of NO.sub.2 to NO. The existence of other substances (in particular carbon dioxide) is obstructive to the emission of light by reason of absorption of a portion of the luminous energy and resultant decrease in the intensity of the emitted light. Since such an unfavorable influence can be reduced by carrying out the aforementioned reaction under very low pressure, the analyzer includes a vacuum pump to maintain a sufficiently low pressure in the reaction chamber.
It is inevitable that the chemiluminescence analyzer including the above described components occupies a large space, and a large amount of electric power is needed to operate those components. Therefore, the analyzer is unsuitable for installation on automobiles or other vehicles though it is useful in bench test.